1. Field of the Invention
This invention relates generally to a write clock pulse generator used for a time base corrector (TBC) and more particularly to a write clock pulse generator used for an apparatus for recording and/or reproducing a video signal which is time-compressed and multiplexed.
2. Description of the Prior Art
In a prior art video tape recorder (VTR), component signals, such as a luminance signal, color difference signals and so on forming a video signal are time-compressed, frequency-multiplexed as shown in FIG. 1A and then recorded, while upon reproducing, the time-compressed and frequency-multiplexed signals are divided, time-expanded and then generated as a reproduced video signal.
In such VTR, the frequency of a write clock pulse used for carrying out the time-compressing and multiplexing upon recording mode and the frequency of a write clock pulse used for carrying out the time-dividing-and-expanding upon reproducing mode are made the same as each other, whereby the respective reproduced component signals are synchronized with one another.
However, the fact that the frequency of the clock signal upon recording and the frequency of the clock signal upon reproducing are made the same as each other means that the frequency of the clock signal upon reproducing is restricted by the frequency of the clock signal upon recording. In practice, if circuits formed of ICs and the like having the same design concept are applied to VTRs of different specifications, the apparatus can be produced with ease and at low manufacturing cost. Accordingly, it is desirable to use the same circuit even when the frequency of the clock signal upon recording and the frequency of the clock signal upon reproducing are different.
When a digital signal is processed by the circuit at the reproducing side (for example, the digital signal is balance-modulated), in most of the circuits, a clock signal having a frequency of a multiple of a color subcarrier (for example, 4 times) is employed. If the frequency of the clock signal used for the time division and expansion upon playback is selected to be the same as the frequency of the clock signal for the digital processing, there will be brought about many advantages in circuit arrangement and signal processing. Accordingly, even if the frequency of the clock signal used in the time division and expansion upon reproducing is different from the frequency of the clock signal used in time-compressing and multiplexing upon recording, consideration should be given to making the frequency of the clock signal used for time division and expansion upon reproducing coincide with the mentioned multiple of the frequency of the color subcarrier.
However, if a frequency N.sub.1 f.sub.H (for example, 720f.sub.H and f.sub.H is the horizontal scanning frequency) of a clock signal RECCK (FIG. 1B) upon recording is made different from a frequency N.sub.2 f.sub.H (for example, 910f.sub.H) of a clock signal PBCK (FIG. 1C) upon reproducing, when the phases of the clock signals RECCK and PBCK are made coincident at the beginning of a component signa C.sub.1 (for example, a color difference signal R-Y) which is time-compressed to the former half H/2 as, for example, shown in FIG. 1A, the phases of the clock signals RECCK and PBCK do not remain coincident at the beginning of a component signal C.sub.2 (for example, a color difference signal B-Y) which is time-compressed to the latter half H/2. As a result, the time expanded and then composed video signal causes a color displacement relative to the original video signal. For instance, when the first component signal C.sub.1 is the color difference signal R-Y and the second component signal C.sub.2 is the color difference signal B-Y, a complete color signal can not be obtained due to the above mentioned color displacement.